A solar cell module typically contains a surface layer, a sealant layer that seals a solar cell, and a backsheet. A copolymer of ethylene and vinyl acetate (also referred to herebelow as EVA) is typically used as the sealant that forms the sealant layer.
A variety of properties, such as mechanical strength, weathering resistance, waterproofness and moistureproofness, electrical insulation, and so forth, are required of the backsheet. Backsheets typically have a multilayer structure and, for example, may contain—considered in sequence from the side in contact with the sealant layer of the solar cell—a weathering-resistant layer/an electrically insulating layer/a waterproof and moistureproof layer/a back layer disposed at the back side of the solar cell.
A film of polyvinyl fluoride is generally used for this weathering-resistant layer and back layer because polyvinyl fluoride exhibits an excellent weathering resistance, waterproofness and moistureproofness, and electrical insulation behavior, while a PET film is generally used for the substrate sheet. In those instances where a high waterproofing and moistureproofing effect is required of the backsheet, a PET film bearing a vapor-deposited metal compound, e.g., silica, or a metal layer, e.g., an aluminum foil, is provided at the surface of the substrate sheet.
In order to satisfy various required properties, e.g., durability, light-blocking ability, and so forth, a backsheet typically has a thickness of 20 to 500 μm. However, there has been demanded in recent years for a reduction in backsheet weight and thickness.
As a consequence, the formation of the same layers using resin coating materials in place of resin sheets has been proposed. For example, the use of an epoxy resin coating material as a resin coating material has been investigated (refer, for example, to Patent Literature 1). However, epoxy resin coating materials have not been realized at a practical level because their cured coating films have an unsatisfactory weathering resistance.
In addition, a two-layer backsheet has been proposed in which a metal substrate (a water-impermeable sheet) is coated with a PVdF-based coating material provided by the incorporation of a specific amount of a tetraalkoxysilane or its partial hydrolyzate into functional group-free PVdF (refer, for example, to Patent Literature 2). Considering this PVdF-based coating material, PVdF, because it lacks functional groups, by itself also has a poor adhesiveness for the EVA used as the sealant. Patent Literature 2 seeks to improve upon this point through the incorporation of a specific amount of a tetraalkoxysilane or partial hydrolyzate thereof and inducing the orientation of the tetraalkoxysilane or partial hydrolyzate thereof at the interface with the EVA.
A solar cell module backsheet has also been proposed in which a cured coating film from a curable functional group-containing fluorinated polymer coating material is formed on at least one side of a water-impermeable sheet (refer, for example, to Patent Literature 3). A curable tetrafluoroethylene (TFE)-based copolymer (Zeffle GK570) is disclosed as the curable functional group-containing fluorinated polymer. Patent Literature 3 teaches that, through the use of this curable functional group-containing fluorinated polymer coating material, the thickness of the backsheet can be reduced in comparison to the conventional application of a sheet and a reduction in thickness and weight can thus be devised while maintaining the mechanical strength, and that, due to the introduction of a functional group into the fluorinated polymer, the adhesiveness to the water-impermeable sheet can be improved even without the addition of, for example, a tetraalkoxysilane.
A solar cell module backsheet has also been proposed in which a cured coating film layer is formed, on one side or both sides of a substrate sheet, from a coating material that contains a fluorinated polymer (A) that has a repeat unit based on (a) a fluoroolefin, a repeat unit based on (b) a crosslinking group-containing monomer, and a repeat unit based on (c) an alkyl group-containing monomer in which a polymerizable unsaturated group is connected by an ether bond or ester bond to a C2-20 straight-chain or branched alkyl group that does not contain the quaternary carbon atom (refer, for example, to Patent Literature 4). Patent Literature 4 teaches that such a cured coating film layer has a particularly good flexibility and a particularly good adherence to the substrate and is free of the problems of cracks, breakage, whitening, and debonding and that a lightweight, high-productivity solar cell module backsheet is obtained.